It was one of the most audacious art thefts seen in Rome: one night in 1971 a gang of thieves slipped into the plush residence of a construction magnate in the upmarket Parioli neighbourhood and walked out with 42 rare paintings, including works by Van Dyck and Poussin.

Detectives soon concluded that the culprits were not members of the mafia, but beyond that, there were few concrete clues, and within days the trail had gone cold.

Now, four decades on, Italian police have recovered some of the stolen paintings from a house in the same district of the country's capital, where they were hanging proudly on the walls. As well as a Van Dyck portrait of a cavalier and a Poussin depiction of the baptism of Christ, the haul contains a Christ on the cross attributed to Rubens, now being verified by experts.

Italian painters Berlinghieri and Guido Reni are also included in the collection, which was described as an exceptional find by Rossella Vodret, an expert from the Italian culture ministry.

The original owner has long since died, but his son and daughter – now adults – were called in to identify the paintings, which they had last seen as young children – and which today are worth around €7.5m (£6.3m).

"When they saw them, they almost fainted," said Colonel Raffaele Mancino, a member of a 300-strong unit of the carabinieri military police which investigates the theft of art and artefacts.

Mancino said the trail to the stolen works – and his unit's biggest seizure in recent years – was discovered when an officer spotted four of the paintings in an auction catalogue and cross-checked them with his database of stolen paintings. Police traced the owner – a 50-year-old woman – and found the four paintings, as well as seven others, hanging in her home.

"She and her husband bought them on the clandestine market 20 years ago and she must have known they were stolen," he said. The woman, who decided to sell the paintings after the recent death of her husband, is now accused of receiving stolen goods.

"She forgot to mention to us she had another house outside Rome, which we checked and found another 26 stolen paintings stacked up, meaning just five are still missing," said Mancino. "This shows we don't give up, even after 41 years."

The identity of the original thieves remains a mystery, and Mancino said such daring thefts were now rare. Instead of gentleman thieves and cunning cat burglars, experts believe the business is now dominated by organised criminal gangs.

"It is a misnomer to see it as an elite crime, since it may trickle down into drugs and weapons," said MaryKate Cleary, a research manager at the London-based Art Loss Register.

Art theft is the third highest grossing criminal business after drugs and weapons, and only about 10% of stolen art is recovered. Traffickers tend to sell works on for 10% of their real value, but gangs often use paintings as collateral in big drug trades owing to the difficulty of moving cash through banks without being traced. "The latest twist, which we have seen in the Balkans, is gangs using fakes of stolen paintings as collateral," said Cleary.

Mancino said the discovery of the 37 paintings in Rome ranked with some of the unit's biggest busts, including the seizure of 10 paintings dating from the 16th century worth €4m that were stolen from the Italian state in 2004.

"We found them in a camper van in Rome where they had been packed in newspaper ready to be driven out of the country," said Mancino. "The driver was asleep."

Not all thefts are undertaken by cross-border criminal gangs, said Mancino, particularly archaeological treasures robbed from digs.

"We have found sarcophagi being used as flower pots, an item from an Etruscan funeral rite being used as a cat's dish and a Roman inscription on marble propping open a window," he said.